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Genetic Crosses With Sex Linked Traits

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In J.R.R Tolkien's The Lord of the Rings, the Black Riders of Mordor ride steeds with eyes of fire. As a geneticist, you are very interested in the inheritance of the fire-red eye colour. A discovery is made that the eyes contains two types of pigments, brown and red, that are usually bound to core granules in the eye. In wild-type steeds, precursors are converted by these granules to the above pigments, but in steeds homozygous for the recessive X-linked gene w (white eye), the granules remain unconverted and a white eye results. The metabolic pathways for the synthesis of the two pigments are shown in the attached file. Each step of the pathway is controlled by a gene: mutation v results in vermillion eyes; cn results in cinnabar eyes; st results in scarlet eyes; bw results in brown eyes; and se results in black eyes. All these mutations are recessive to their wild-type alleles, and all are unlinked. For the following genotypes, show the proportions of steed eye phenotypes that would be obtained in the F1 of the given matings.

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Genetic crosses are depicted.

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The 'Lord of the Rings' questions deal with genetic crosses, and these type of questions can all be done in the same way. It is easiest to draw it out for yourself. You have been provided with a pathway that you must keep in mind and this cross also involves a sex-linked trait, a bit of a curveball but no big deal.

First thing is to determine what all the traits will give you. You have been told that these genes aren't linked (meaning they are very far apart on a chromosome, or lie on different chromosomes) so you can deal with each one individually. To get a functional gene product, all we need is one dominant allele. With these points in mind, we have been given the following information.

bw : brown eyes (first step in red pigment pathway)
se : black eyes (last step in red pigment pathway)

v : vermillion eyes (first step in brown pathway)
cn : cinnabar eyes (second step in brown pathway)
st : scarlet eyes (last step in brown pathway)

In order to get the fire eyes, we must have all these genes functional (dominant). We were also given information about a sex-linked gene w+. We must have a functional w, or they will end up ...

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